Wireless power transmitter having low noise and high efficiency, and related methods

ABSTRACT

A wireless power transmitter comprises a bridge inverter including a first switch and a second switch coupled together with a first switching node therebetween, and a first capacitor coupled to the first switching node. The transmitter further includes control logic configured to control the first switch and the second switch according to an operating frequency to generate an AC power signal from a DC power signal, and a resonant tank operably coupled to the first switching node of the bridge inverter, the resonant tank configured to receive the AC power signal and generate an electromagnetic field responsive thereto. A method for operating the wireless power transmitter and a method for making the wireless power transmitter are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/724,841, filed Nov. 9, 2012, entitled “Low NoiseHigh Efficiency Power Stage,” docket number IDT-2916PR, the disclosureof which is hereby incorporated herein by this reference in itsentirety.

FIELD

Embodiments of the present disclosure relate generally to wireless powertransfer and, more particularly, to apparatuses and methods related to awireless power transmitter.

BACKGROUND

Battery-powered devices (e.g., consumer electronic devices, electric andhybrid automobiles, etc.) are charged from a power source (e.g., ACpower outlet) through a charging device. The charging device couples thebattery to the power source through an adaptor. The cord extendingbetween the power source and the battery-powered device can take upspace. In situations where multiple devices require charging, each withtheir own charger and cord, the charging area can become cramped andinconvenient.

Approaches are being developed that use over-the-air or wireless.powertransmission between a transmitter and a receiver coupled to theelectronic device. Wireless power transmission using inductive coils isone method considered as an un-tethered method for transferring powerwirelessly through a coupled electromagnetic field. In wireless powertransmission, power is transferred by transmitting an electromagneticfield through a transmit coil. On the receiver side, a receive coil maycouple with the transmit coil through the electromagnetic field, thus,receiving the transmitted power wirelessly. The distance between thetransmit and receive coils, at which efficient power transfer can takeplace, is a function of the transmitted energy and the requiredefficiency. The coupling coefficient (k) is a function of the distancebetween the coils, the coil sizes, and materials. The power conversionefficiency (e.g., coupling factor, coupling quality) may besignificantly improved if the coils are sized and operated at such afrequency that they are physically within the so-called “near-fieldzone” of each other.

Wireless power systems are generally intended to operate in a frequencyrange substantially near (e.g., exactly at) the peak resonance of theresonant tanks of the wireless power devices. The operating frequency ofa wireless power transmitter may be determined by the switchingfrequency of the gate drives of the bridge inverter used to convert a DCsignal to the AC signal used to generate the wireless power signal. Thefaster the gates of the switches are switched, the faster the rate ofchange (dv/dt) of the switching voltage exists on the switching nodesbetween the switches. An electrical node that has a relatively fast rateof change (dv/dt) may capacitively couple easily to surroundingcircuitry. Through such parasitic capacitive coupling, the faster therate of change (dv/dt), the more current will flow in undesirable places(e.g., the leads of the wireless power transmitter, other components ofthe system, etc.). As a result, electromagnetic interference (EMI) maybe introduced into the system. Conventional wireless power transmittersmay introduce filters between the switch controller and the gates of theswitches to slow down the gate drives to the switches of the bridgeinverter. Slowing down the rate of change (dv/dt) for the gate drives ofthe switches may come at the expense of power loss in the switches.

Thus, in a wireless power transfer power stage there are oftentimesconflicting requirements for low noise emissions and high operationalefficiency. Low noise emissions may require the switches of aconventional power transfer stage to switch relatively slowly such thatthe rate of change (dv/dt) of the switching voltages are not very high,which may come at the expense of power loss in the switches—both whenbeing enabled and disabled. On the other hand, high efficiency of aconventional power transfer stage may require that the switches areswitched relatively fast, but which may result in fast rate of change ofthe switching voltages and increased noise.

BRIEF SUMMARY

Embodiments of the present disclosure include a wireless powertransmitter. The wireless power transmitter comprises a bridge inverterincluding a first switch and a second switch coupled together with afirst switching node therebetween, and a first capacitor coupled to thefirst switching node. The wireless power transmitter further comprisescontrol logic configured to control the first switch and the secondswitch according to an operating frequency to generate an AC powersignal from a DC power signal, and a resonant tank operably coupled tothe first switching node of the bridge inverter, the resonant tankconfigured to receive the AC power signal and generate anelectromagnetic field responsive thereto.

Another embodiment of the present disclosure includes a method ofoperating a wireless power transmitter. The method comprises operating afirst switch and a second switch of a bridge inverter according to anoperating frequency to generate an AC signal from a DC signal, the firstswitch and the second switch having a first capacitor coupled at a firstswitching node therebetween. The method further comprises generating awireless power signal through a resonant capacitor and a transmit coilcoupled to the first switching node.

Another embodiment of the present disclosure includes a method of makinga wireless power transmitter. The method comprises coupling at least onecapacitor to at least one switching node of a bridge inverter of awireless power transmitter, and coupling at least one resonant capacitorand at least one transmit coil to the at least one switching node.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a wireless power transfer systemaccording to an embodiment of the present disclosure.

FIG. 2 is a schematic block diagram of a wireless power transfer systemaccording to an embodiment of the present disclosure.

FIGS. 3A-3D are schematic diagrams of various configurations of a bridgeinverter for a wireless power transmitter according to an embodiment ofthe present disclosure.

FIGS. 4A, 4B are schematic diagrams of are schematic diagrams of variousconfigurations of a bridge inverter for a wireless power transmitteraccording to an embodiment of the present disclosure.

FIGS. 5A, 5B are waveforms illustrating the operation of the switches ofthe bridge inverter of a wireless power transmitter according to anembodiment of the present disclosure.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings in which is shown, by way of illustration, specific embodimentsof the present disclosure. Other embodiments may be utilized and changesmay be made without departing from the scope of the disclosure. Thefollowing detailed description is not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

Furthermore, specific implementations shown and described are onlyexamples and should not be construed as the only way to implement orpartition the present disclosure into functional elements unlessspecified otherwise herein. It will be readily apparent to one ofordinary skill in the art that the various embodiments of the presentdisclosure may be practiced by numerous other partitioning solutions.

In the following description, elements, circuits, and functions may beshown in block diagram form in order not to obscure the presentdisclosure in unnecessary detail. Additionally, block definitions andpartitioning of logic between various blocks is exemplary of a specificimplementation. It will be readily apparent to one of ordinary skill inthe art that the present disclosure may be practiced by numerous otherpartitioning solutions. Those of ordinary skill in the art wouldunderstand that information and signals may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof. Some drawings may illustrate signals as a single signal forclarity of presentation and description. It will be understood by aperson of ordinary skill in the art that the signal may represent a busof signals, wherein the bus may have a variety of bit widths and thepresent disclosure may be implemented on any number of data signalsincluding a single data signal. In addition, elements of a wirelesspower transmitter (including a bridge inverter) may be implemented usingdiscrete components, as an integrated circuit (IC), or combinationsthereof.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general-purpose processor, a special-purposeprocessor, a Digital Signal Processor (DSP), an Application-SpecificIntegrated Circuit (ASIC), a Field-Programmable Gate Array (FPGA) orother programmable logic device, a controller, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A general-purposeprocessor may be considered a special-purpose processor while thegeneral-purpose processor executes instructions (e.g., software code)stored on a computer-readable medium. A processor may also beimplemented as a combination of computing devices, such as a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

Also, it is noted that the embodiments may be described in terms of aprocess that may be depicted as a flowchart, a flow diagram, a structurediagram, or a block diagram. Although a process may describe operationalacts as a sequential process, many of these acts can be performed inanother sequence, in parallel, or substantially concurrently. Inaddition, the order of the acts may be re-arranged. A process maycorrespond to a method, a function, a procedure, a subroutine, asubprogram, etc. Furthermore, the methods disclosed herein may beimplemented in hardware, software, or both. If implemented in software,the functions may be stored or transmitted as one or more instructionsor code on computer readable media. Computer-readable media includesboth non-transitory computer storage media and communication media,including any medium that facilitates transfer of a computer programfrom one place to another.

It should be understood that any reference to an element herein using adesignation such as “first,” “second,” and so forth does not limit thequantity or order of those elements, unless such limitation isexplicitly stated. Rather, these designations may be used herein as aconvenient method of distinguishing between two or more elements orinstances of an element. Thus, a reference to first and second elementsdoes not mean that only two elements may be employed or that the firstelement must precede the second element in some manner. In addition,unless stated otherwise, a set of elements may comprise one or moreelements.

Embodiments of the disclosure may include apparatuses, systems, andmethods related to a wireless power transmitter. In particular, thewireless power transmitter may comprise a bridge inverter including acapacitor coupled to a switching node between the switches of the bridgeinverter. The wireless power transmitter may further include controllogic configured to control the switches of the bridge inverteraccording to a desired break-before-make timing. As a result, thevoltage level at the switching node may exhibit a relatively slow rateof change while improving efficiency of the wireless power transmitterby reducing switching losses of the switches compared with conventionalwireless power transmitters.

FIG. 1 is a schematic block diagram of a wireless power transfer system100 according to an embodiment of the present disclosure. The wirelesspower transfer system 100 includes a wireless power transmittingapparatus 110, and a wireless power receiving apparatus 120. Thewireless power transmitting apparatus 110 includes a wireless powertransmitter 112 having a transmit coil 114 configured to generate anelectromagnetic field 105 for providing power transfer to the wirelesspower receiving apparatus 120. The wireless power receiving apparatus120 includes a wireless power receiver 122 having a receive coil 124configured to couple with the electromagnetic field 105. The transmitcoil 114 and the receive coil 124 may be sized according to theparticular devices and applications to be associated therewith. Theelectromagnetic field 105 may also be referred to as the wireless powersignal 105 for power transfer from the wireless power transmitter 112 tothe wireless power receiver 122.

An input signal 116 may be provided to the wireless power transmitter112 for providing the wireless power transmitter 112 with the power forgenerating the wireless power signal 105 that provides a power transferto the wireless power receiving apparatus 120. The wireless powerreceiver 122 may couple to the wireless power signal 105 and generate anoutput signal 126 in response thereto. The output signal 126 may providethe power that is used by the wireless power receiving apparatus 120 forstoring (e.g., charging a battery), consumption (e.g., providing systempower), or both.

The wireless power transmitter 112 and the wireless power receiver 122are separated by a distance (d). In some embodiments, the wireless powertransmitter 112 and the wireless power receiver 122 may be configuredaccording to a mutual inductance relationship, such that when theresonant frequency of the wireless power receiver 122 and the resonantfrequency of the wireless power transmitter 112 are substantiallyidentical, transmission losses between the wireless power transmitter112 and the wireless power receiver 122 are minimal. Likewise, thefrequency of the wireless power signal 105 may be set by the wirelesspower transmitter 112 at or near the resonant frequencies of the coils114, 124. As a result, an efficient power transfer occurs by coupling alarge portion of the energy in the near-field of the transmit coil 114to the receive coil 124 rather than propagating most of the energy in anelectromagnetic wave to the far-field. If the wireless power receivingapparatus 120 is in the near-field (within some distance (d)), inductivecoupling may occur between the transmit coil 114 and the receive coil124. The area around the transmit coil 114 and receive coil 124 wherethis near-field inductive coupling may occur may be referred to as a“coupling region.” Because of this mutual inductance relationship, thewireless power transfer may be referred to as inductive wireless powertransfer.

The transmit coil 114 and the receive coil 124 may be configured as a“loop” antenna, which may also be referred to herein as a “magnetic”antenna or an “inductive” antenna. Loop antennas may be configured toinclude an air core or a physical core such as a ferrite core. Air coreloop antennas may be more tolerable to extraneous physical devicesplaced in the vicinity of the core. Furthermore, an air core loopantenna allows the placement of other components within the core area.In addition, an air core loop may more readily enable placement of thereceive coil 124 within a plane of the transmit coil 114 where thecoupling region of the transmit coil 114 may be more powerful.

The wireless power transmitting apparatus 110 may include a wirelesspower transmitter to transmit the wireless power signal 105. Thewireless power receiving apparatus 120 may be a mobile electronicdevice, such as a cell phone, a smart phone, a media player (e.g., mp3player, DVD player, etc.), an electronic reader, a tablet computer, apersonal digital assistant (PDA), a camera, a laptop computer, andpersonal electronic device in which wireless power signal 105 may bereceived. The wireless power receiving apparatus 120 may also be a lessmobile electronic device, such as a television, personal computer, mediaplayer (e.g., DVD player, Blu-ray player, etc.) or any other device thatmay operate by, and/or store electrical power. The wireless powerreceiving apparatus 120 may be one of a number of other items, such asan automobile or any other devices that may include batteries that maybe charged through the wireless power transmitting apparatus 110.

The wireless power transmitting apparatus 110 may be a device that may,at times, also be the recipient of wireless power transfer. In otherwords, some devices may be configured as both a wireless powertransmitting apparatus 110 and a wireless power receiving apparatus 120,such that the device may transmit wireless power or receive wirelesspower depending on the mode of operation. Thus, embodiments of thepresent disclosure include devices that may include a wireless chargingtransceiver configured to operate in either a transmit mode or a receivemode. Using the tenn “receiver” indicates that a device is configured toreceive wireless power transfer, but should not be interpreted to meanthat the device only operates as a receiver. Similarly, using the term“transmitter” indicates that the device is configured to transmitwireless power, but should not be interpreted to mean that the deviceonly operates as a transmitter.

FIG. 2 is a schematic block diagram of a wireless power transfer system200 according to an embodiment of the present disclosure. The wirelesspower transfer system 200 includes a wireless power transmitter 212having a resonant tank 213 that includes a transmit coil 214 coupledwith resonant capacitors 215. The wireless power transmitter 212includes a bridge inverter 217 and control logic 218 coupled with theresonant tank 213. The bridge inverter 217 of the wireless powertransmitter 212 may include a full-bridge inverter, a half-bridgeinverter, or other appropriate circuit for receiving a DC input signal216 and generate an AC signal through the transmit coil 114 forgenerating the wireless power signal 105. The wireless power receiver222 includes a resonant tank 223 having a receive coil 224 coupled withresonant capacitors 230. The resonant capacitors 230 are coupled with arectifier 250. The wireless power transmitter 212 and the wireless powerreceiver 222 may be incorporated within a wireless power transmittingapparatus 110 (FIG. 1) and a wireless power receiving apparatus 120(FIG. 1), respectively. The transmit coil 214 and the receive coil 224(and other components) may be sized according to the particular devicesand applications to be associated therewith.

The control logic 218 of the wireless power transmitter 212 may beconfigured to control one or more operations of the wireless powertransmitter 212. Similarly, the control logic 280 of the wireless powerreceiver 222 may be configured to control one or more operations of thewireless power receiver 222. Each of the control logic 218, 280 may beimplemented within a processor (e.g., microcontroller) or othercircuitry that is configured (e.g., programmed) to perform variousoperations of embodiments of the present disclosure. Each of the controllogic 218, 280 may further include computer-readable media (e.g.,memory) storing computing instructions for execution by the processorrelated to performing processes of the embodiments of the presentdisclosure. Memory may include volatile and non-volatile memory.

The wireless power transmitter 212 may be configured to generate awireless power signal 105 (FIG. 1) responsive to a DC input signal 216received by the resonant tank 213. The wireless power receiver 222 maybe configured to couple with the wireless power signal 150 (whichinduces an AC current in the receive coil 224) and generate an outputpower signal (a DC signal) to provide power to a load 270. As a result,the output power signal may include a rectified voltage (V_(RECT)) andrectified current (I_(RECT)) that is provided to the load 270. The load270 may include an energy storage device (e.g., battery, such as alithium-ion battery), system components of a wireless power enableddevice, or a combination thereof. Thus, the output signal from thewireless power receiver 222 may be used for charging an energy storagedevice and/or for providing system power to the various systemcomponents of a device.

The wireless power transmitter 212 and wireless power receiver 222 maybe generally configured as discussed above with respect to FIG. 1. Theconfigurations of the LC networks within the resonant tanks 213, 223 maygenerally determine the resonant frequencies of the wireless powertransmitter 212 and the wireless power receiver 222, respectively. Forexample, the resonant frequency of the resonant tanks 213, 223 may bebased on the inductance of their respective inductive coil and thecapacitance of the plates of the capacitors. The wireless powertransmitter 212 and the wireless power receiver 222 may be configuredaccording to a mutual inductance relationship, such that when theresonant frequency of the wireless power receiver 222 and the resonantfrequency of the wireless power transmitter 212 are substantiallyidentical, transmission losses between the wireless power transmitter212 and the wireless power receiver 222 are minimal. As a result, thecoupling efficiency and wireless power transfer may be improved. Inaddition, the frequency of the wireless power signal 105 may be set bythe wireless power transmitter 212 at or near the resonant frequenciesof the wireless power transmitter 212 and wireless power receiver 222for minimal transmission losses.

During wireless power transmission, the input signal 216 (a DC signal)may be received by the bridge inverter 217. The bridge inverter 217 maygenerate an AC current that flows through the resonant tank 213 togenerate a time-varying electromagnetic field for transmitting thewireless power signal 105. Thus, the wireless power signal 105 may be atime-varying signal that is substantially sinusoidal, having a frequencythat may be based on the switching frequency of the bridge inverter 217of the wireless power transmitter 212. In some embodiments, thefrequency of the wireless power signal 105 may be set according to thedesired frequency, such as a frequency for a particular wireless powerstandard. The resonant tank 213 may be configured such that the resonantfrequency is approximately the frequency of the wireless power signal105. In some embodiments, it may be desirable for the frequency of thewireless power signal 105 to differ somewhat from the resonant frequencyof the resonant tank 213, such as to reduce the peak-to-peak currentthrough the transmit coil 214.

In order to receive wireless power signal 105, the wireless powerreceiver 222 may be placed in the coupling region of the wireless powertransmitter 212 such that inductive coupling may be achieved. As aresult, the wireless power receiver 222 may receive the wireless powersignal 105 and generate an AC power responsive thereto. In order for thepower to be used by the load 270, the AC power may be converted to a DCpower. The rectifier 250 may generate a rectified voltage (V_(IcT)) aswell as a rectified current (I_(REcT)) flowing through the resonant tank223. In some embodiments, the rectifier 250 may be configured as asynchronous rectifier. As a result, the rectifier 250 may include one ormore switches that are controlled in such a manner to generate the DCoutput power signal (i.e., rectified voltage (V_(RECT)) and rectifiedcurrent (V_(RECT))). In some embodiments, the rectifier 250 may includeone or more diodes configured to generate the DC output power signal.

The wireless power transmitter 212 may be configured to control thebridge inverter 217 to set the switching frequency for the desiredoperating frequency. In addition, each of the control logic 218, 280 mayfurther control other functions of the respective wireless powertransmitter 212, wireless power receiver 222, such as controls relatedto modulation/demodulation, foreign object detection, device operation,etc. The control logic 218, 280 may each include different sub-blocksthat perform one or more of the above functions separately rather thanby employing within a single process, routine, program, etc. Inaddition, the control logic 218, 280 may each employ different hardwareelements for different functions.

It should be recognized that the devices of a wireless power transfersystem 200 may include additional components to perform other featuresnot specifically described herein or shown in the various figures. Forexample, wireless power enabled devices may include a modulator and/or ademodulator for communicating with other devices, foreign objectdetection modules, I/O modules for interfacing with a user, memory forstoring instructions and data, various sensors, processors, controllers,voltage regulators, among other components. The figures and accompanyingdescription may, therefore, be somewhat simplified to focus on thevarious apparatuses and methods that are configured to decrease the rateof change (dv/dt) of the switching voltages of the wireless powertransmitter.

FIGS. 3A-3D are schematic diagrams of various configurations of a bridgeinverter 217 for a wireless power transmitter 212 (FIG. 2) according toan embodiment of the present disclosure. The bridge inverter 217 may beconfigured to receive and convert a DC input signal 216 to an AC signalto flow through the resonant tank 213 that generates the wireless powersignal 105. The resonant tank 213 includes at least one resonantcapacitor 215 and the transmit coil 214, which may be coupled in series,and which may be the primary contributors to the resonant frequency ofthe wireless power transmitter 212.

The bridge inverters 217 of FIGS. 3A-3D are configured as full-bridgeinverters that include switches 302, 304, 306, 308 coupled to theresonant tank 213. The switches 302, 304 may be configured as powerMOSFETs or other suitable transistors. The switches 302, 304 may beserially coupled on one side of the resonant tank 213, and switches 302,304 may be serially coupled on the other side of the resonant tank 213.In an example, the drain of the high-side switch 302 may be coupled to asupply voltage (i.e., the input voltage V_(IN)), and the source of thelow-side switch 304 may be coupled to ground. The source of thehigh-side switch 302 and the drain of the low-side switch 304 may becoupled at the switching node. The high-side switch 306 and the low-sideswitch 308 on the other side of the bridge inverter 217 may be similarlycoupled. The high-side switches 302, 306 may include p-channel (e.g.,PMOS) transistors that are enabled when the gate drive voltages V_(G1),V_(G4) are low. The low-side switches 304, 308 may include n-channel(e.g., NMOS) transistors that are enabled if the gate drive voltagesV_(G2), V_(G3) are high. Of course, other combinations of p-channel andn-channel transistors are contemplated for the switches 302, 304, 306,308.

The resonant tank 213 may be coupled to the switching nodes (labeled bytheir switching voltages V_(SW1), V_(SW2) on the nodes) that are locatedbetween the respective switches 302, 304, 306, 308. The switches 302,304, 306, 308 may be controlled by the control logic 218 (FIG. 2). Inparticular, the control logic 218 (e.g., a switching controller) maytransmit control signals (i.e., gate drive voltages V_(G1), V_(G2),V_(G3), V_(G4)) to alternatingly enable and disable the switches 302,304, 306, 308 according to a desired operating frequency (i.e.,switching frequency). In other words, switches 302, 304 may becomplementary enabled with each other, while switches 306, 308 may becomplementary enabled with each other. As an example, the operatingfrequency may depend on a standard for which the wireless powertransmitter 212 is desired to operate. Exemplary wireless powerstandards include the Wireless Power Consortium (WPC), the Power MattersAlliance (PMA), and the Alliance for Wireless Power (A4WP).

The bridge inverter 217 may further include capacitors 390, 392 that arecoupled at the switching nodes between the switches 302, 304, 306, 308.In particular, the first capacitor 390 is coupled to the switching nodebetween the switches 302, 304, and the second capacitor is coupled tothe switching node between the switches 306, 308.

In operation, the switches 302, 304, 306, 308 are complementary enabledand disabled according to a desired operating frequency. The faster thegates of the switches 302, 304, 306, 308 are switched, the faster therate of change (dv/dt) of the switching voltage V_(SW1), V_(SW2) existson the switching nodes. As discussed above, an electrical node that hasa relatively fast rate of change (dv/dt) may capacitively couple easilyto surrounding circuitry, which may introduce EMI and noise into thesystem. Thus, rather than slowing down the rate of change (dv/dt) of thegate drive voltages V_(G1), V_(G2), V_(G3), V_(G4), the capacitors 390,392 slow down the rate of change (dv/dt) of the switching voltagesV_(SW1), V_(SW2) on the switching nodes between the switches 302, 304,306, 308. As a result, the wireless power transmitter 212 may experiencerelatively low noise from capacitive coupling to the surroundingcircuitry, while reducing the power losses typically incurred byconventional wireless power transmitters.

During switching, the high-side switch 302 is disabled and the low-sideswitch 304 is disabled. Because the switches 302, 304 are physicaldevices (i.e., not ideal), the switching does not occur instantaneous.If the high-side switch 302 and the low-side switch 304 were at leastpartially enabled at the same time, there may be a short between thepower supply and ground. As a result, the switches 302, 304 or othercomponents may be damaged by an uncontrolled current flow between thetwo terminals. In order to avoid this situation, the control logic 218may be configured to add a “break-before-make” time into the timing ofdriving the gate drive voltages V_(G1), V_(G2), V_(G3), V_(G4). Thebreak-before-make time is an amount of delay between the time that oneswitch (e.g., high-side switch 302) is disabled and the other switch(e.g., low-side switch 304) is enabled. In order to accommodate theslower rate of change (dv/dt) on the switching nodes, thebreak-before-make time may be increased in comparison to conventionalwireless power transmitters. For example, some conventional wirelesspower transmitters may have a break-before-make time betweenapproximately 10 ns to 20 ns, whereas embodiments of the presentdisclosure may be an order of magnitude higher (e.g., betweenapproximately 150 ns to 200 ns) depending on the particular application.Other ranges of break-before-make times are also contemplated, such as arange between 10 ns to 1 μs. The break-before make timing may bepre-determined or determined on the fly.

The rate of change (dv/dt) of each of the switching voltages V_(SW1),V_(SW2) depends on the value of the capacitors 390, 392 and the amountof current flowing therethrough. For example, the capacitance of each ofthe capacitors 390, 392 may be in the range of 100 pF to 100 nF;however, other values are contemplated. The current is typically storedin the transmit coil 214, which acts as a pseudo current source. Eachtime the switches 302, 304, 306, 308 are enabled and disabled, energy isstored in the transmit coil 214. During the time that both switches 302,304 are disabled, energy is pumped back into the capacitor 390. Over aperiod of time, the switching voltage V_(SW1) increases toward thesupply voltage V_(IN). The amount of time for the switching voltage toincrease to approximately equal the supply voltage may be known if thecurrent and the capacitance of the capacitor 390 may be known.Therefore, the control logic 218 may implement a break-before-make timethat is at least as long as the amount of time for the switching voltageV_(SW1) to increase to approximately equal the supply voltage V_(IN),which allows the switching voltage V_(SW1) to naturally slew up and slewdown at the desired slow rate. Although the operation of the switches302 and 304 is used as an example to describe the operation of thebridge inverter 217, the operation of switches 306, 308 may be performedsimilarly.

Referring individually to the various configurations of FIGS. 3A-3D, theplacement of the capacitors 390, 392 may vary. For example, as shown inFIG. 3A the first capacitor 390 may be coupled between the firstswitching node and ground, and the second capacitor 392 may be coupledbetween the second switching node and ground. As shown in FIG. 3B, thefirst capacitor 390 may be coupled between the first switching node andground, and the second capacitor 392 may be coupled between the secondswitching node and the supply voltage V_(IN). As shown in FIG. 3C, thefirst capacitor 390 may be coupled between the first switching node andthe supply voltage V_(IN), and the second capacitor 392 may be coupledbetween the second switching node and the supply voltage V_(IN). Asshown in FIG. 3D, the first capacitor 390 may be coupled between thefirst switching node and the supply voltage V_(IN), and the secondcapacitor 392 may be coupled between the second switching node andground.

FIGS. 4A, 4B are schematic diagrams of are schematic diagrams of variousconfigurations of a bridge inverter 217 for a wireless power transmitter212 (FIG. 2) according to an embodiment of the present disclosure. Inparticular, the bridge inverters 217 of FIGS. 4A, 4B are configured ashalf-bridge inverters that include switches 302, 304 coupled to theresonant tank 213. The switches 302, 304 may be configured as powerMOSFETs or other suitable transistors. The switches 302, 304 may beserially coupled having a switching node therebetween. In an example,the drain of the high-side switch 302 may be coupled to a supply voltage(i.e., the input voltage V_(IN)), and the source of the low-side switch304 may be coupled to ground. The source of the high-side switch 302 andthe drain of the low-side switch 304 may be coupled at the switchingnode. The high-side switch 302 may include a p-channel (e.g., PMOS)transistor that is enabled when the gate drive voltages V_(G1) is low.The low-side switch 304 may include an n-channel (e.g., NMOS) transistorthat is enabled if the gate drive voltage V_(G2) is high. The resonanttank 213 coupled between the switching node of the switches 302, 304 andground.

The bridge inverter 217 may further include the capacitor 390 coupled tothe switching node. In some embodiments, the capacitor 390 may becoupled between the switching node and ground (FIG. 4A). In someembodiments, the capacitor 390 may be coupled between the switching nodeand the supply voltage V_(IN) (FIG. 4B). The operation of the bridgeinverter 217 may operate similarly as described above with respect toFIGS. 3A-3D, but without any switches on the other side of the resonanttank 213.

FIGS. 5A, 5B are waveforms illustrating the operation of the switches302, 304 (switches 306, 308 may operate similarly) of the bridgeinverter 217 of a wireless power transmitter 212 according to anembodiment of the present disclosure. The switches 302, 304 may becomplementary enabled such that the switches 302, 304 alternate beingenabled (i.e., on) and disabled (i.e., off). In particular, FIG. 5Aillustrates the transition of the low-side switch 304 from being enabledto being disabled, and the high-side switch 302 from being disabled tobeing enabled. FIG. 5A, on the other hand, illustrates the opposingaction of the high-side switch 302 transitioning from being enabled tobeing disabled, and the low-side switch 304 transitioning from beingdisabled to being enabled.

Referring specifically to FIG. 5A, at time t₁ the control logic 218deasserts the second gate drive voltage V_(G2) while the first gatedrive voltage V_(G1) remains asserted. Because the low-side switch 304is an n-channel transistor, the low-side switch 304 transitions frombeing enabled to being disabled at time t₁. At time t2, the controllogic 218 deasserts the first gate drive voltage V_(G1) while the secondgate drive voltage V_(G2) remains deasserted. Because the high-sideswitch 302 is a p-channel transistor, the high-side switch 302transitions from being disabled to being enabled at time t₂. Thus, thetime period t_(B) is the break-before-make time set by the control logic218 to ensure that the low-side switch 304 is completely disabled priorto enabling the high-side switch 302. In other words, during thebreak-before-make time, both switches 302, 304 are disabled.

When the low-side switch 304 transitions from being enabled to beingdisabled, the low-side switch 304 ceases to conduct current. As aresult, the current in the bridge inverter 217 is diverted to thecapacitor 390. In addition, the first switching voltage V_(SW1) beginsto rise according to a rate of change (dv/dt) across the capacitor 390.As a result, the rate of change (dv/dt) of the first switching voltageV_(SW1) is relatively slower than conventional wireless powertransmitters. For example, the first switching voltage may reach thesupply voltage V_(IN) in approximately 150 ns to 200 ns in comparisonwith 10 ns to 20 ns for a conventional wireless power transmitter. Thebreak-before make times may also be similar as those times.

Referring specifically to FIG. 5B, at time t₁ the control logic 218asserts the first gate drive voltage V_(G1) while the second gate drivevoltage V_(G2) remains deasserted. Because the high-side switch 302 is ap-channel transistor, the high-side switch 302 transitions from beingenabled to being disabled at time t₁. At time t2, the control logic 218asserts the second gate drive voltage V_(G2) while the first gate drivevoltage V_(G2) remains asserted. Because the low-side switch 304 is ann-channel transistor, the low-side switch 304 transitions from beingdisabled to being enabled at time t₂. Thus, the time period t_(B) is thebreak-before-make time set by the control logic 218 to ensure that thehigh-side switch 302 is completely disabled prior to enabling thelow-side switch 304. During this transition, the switching voltageV_(SW1) decreases toward ground according to a rate of change (dv/dt)across the capacitor 390. Similar to the first transition, the rate ofchange (dv/dt) may be relatively slower than conventional wireless powertransmitters. As a result, a relatively high efficiency, low noise,wireless power transfer stage may be achieved.

While the present disclosure has been described herein with respect tocertain illustrated embodiments, those of ordinary skill in the art willrecognize and appreciate that the present invention is not so limited.Rather, many additions, deletions, and modifications to the illustratedand described embodiments may be made without departing from the scopeof the disclosure. In addition, features from one embodiment may becombined with features of another embodiment while still beingencompassed within the scope of the disclosure as contemplated by theinventor.

What is claimed is:
 1. A wireless power transmitter, comprising: abridge inverter including: a first switch and a second switch coupledtogether with a first switching node therebetween; and a first capacitorcoupled to the first switching node; control logic configured to controlthe first switch and the second switch according to an operatingfrequency to generate an AC power signal from a DC power signal; and aresonant tank operably coupled to the first switching node of the bridgeinverter, the resonant tank configured to receive the AC power signaland generate an electromagnetic field responsive thereto.
 2. Thewireless power transmitter of claim 1, wherein the bridge inverterincludes a full bridge inverter.
 3. The wireless power transmitter ofclaim 2, wherein the bridge inverter further includes: a third switchand a fourth switch coupled together with a second switching nodetherebetween; and a second capacitor coupled to the second switchingnode.
 4. The wireless power transmitter of claim 3, wherein the resonanttank is operably coupled to the second switching node of the bridgeinverter.
 5. The wireless power transmitter of claim 4, wherein theresonant tank includes at least one resonant capacitor operably coupledwith the transmit coil between the first switching node and the secondswitching node.
 6. The wireless power transmitter of claim 3, whereincapacitance for each of the first capacitor and the second capacitor iswithin a range of values from 100 pF to 100 nF.
 7. The wireless powertransmitter of claim 3, wherein: the first capacitor is coupled betweenthe first switching node and ground; and the second capacitor is coupledbetween the second switching node and ground.
 8. The wireless powertransmitter of claim 3, wherein: the first capacitor is coupled betweenthe first switching node and a power supply; and the second capacitor iscoupled between the second switching node and ground.
 9. The wirelesspower transmitter of claim 3, wherein: the first capacitor is coupledbetween the first switching node and a power supply; and the secondcapacitor is coupled between the second switching node and the powersupply.
 10. The wireless power transmitter of claim 3, wherein: thefirst capacitor is coupled between the first switching node and ground;and the second capacitor is coupled between the second switching nodeand a power supply.
 11. The wireless power transmitter of claim 1,wherein the first switch is a p-channel transistor, and the secondswitch is an n-channel transistor.
 12. The wireless power transmitter ofclaim 1, wherein the bridge inverter includes a half-bridge inverter.13. The wireless power transmitter of claim 1, wherein the control logicis further configured to implement a break-before-make time that isgreater than or equal to a rate of change of the first switchingvoltage.
 14. A method of operating a wireless power transmitter, themethod comprising: operating a first switch and a second switch of abridge inverter according to an operating frequency to generate an ACsignal from a DC signal, the first switch and the second switch having afirst capacitor coupled at a first switching node therebetween; andgenerating a wireless power signal through a resonant capacitor and atransmit coil coupled to the first switching node.
 15. The method ofclaim 14, wherein operating the first switch and the second switchincludes disabling the first switch and enabling the second switch attimes that are separated by a break-before-make time.
 16. The method ofclaim 14, wherein operating the first switch and the second switch ofthe bridge inverter includes generating a first switching voltage on thefirst switching node having a rate of change that is responsive tocurrent flowing through the first capacitor.
 17. The method of claim 14,further comprising operating a third switch and a fourth switch of thebridge inverter according to the operating frequency to generate the ACsignal from the DC signal, the third switch and the fourth switch havinga second capacitor coupled at a second switching node therebetween. 18.A method of making a wireless power transmitter, the method comprising:coupling at least one capacitor to at least one switching node of abridge inverter of a wireless power transmitter; and coupling at leastone resonant capacitor and at least one transmit coil to the at leastone switching node.
 19. The method of claim 18, wherein coupling the atleast one capacitor to the at least one switching node includes:coupling a first capacitor to a first switching node of an FET pair; andcoupling a second capacitor to a second switching node of another FETpair.
 20. The method of claim 19, wherein: coupling the first capacitorto the first switching node of an FET pair includes coupling the firstcapacitor between the first switching node and one of a power supplyterminal and ground; and coupling the second capacitor to the secondswitching node of another FET pair includes coupling the secondcapacitor between the second switching node and one of the power supplyterminal and ground.